GRXE 1

The Galactic Ridge X-ray Emission

F.E. Marshall (GSFC)

Elihu Boldt Memorial Symposium November 6, 2009

GRXE 2

Outline of Talk

•  Introduction •  History of GRXE has many similarities to that of CXRB.

•  Early Results from HEAO A2 •  First instrument designed to measure diffuse X-ray emission.

•  Recent Results •  RXTE, Chandra, & Suzaku

•  Future Work

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Introduction

•  X-ray emission from the plane of the Milky Way (Galactic Ridge X-ray Emission -- GRXE) was a long-time interest of E. Boldt. •  His interest was, in part, motivated by his CR research.

•  Boldt & Serlemitsos (1969) discussed X-ray bremsstrahlung from CR protons and speculated on emission in ISM; Pravdo & Boldt (1975) calculated line emission from CR oxygen.

•  In a rocket flight Bleach et al. (1972) discovered GRXE & discussed possible emission mechanisms.

•  Contributors could include discrete sources and diffuse mechanisms such as thermal bremstrahlung, non-thermal bremstrahlung (e & p), inverse Compton, and synchrotron.

•  Subsequent observations with HEAO A2, RXTE, Suzaku, and other missions have greatly increased our knowledge, but the source(s) of the GRXE remains controversial.

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HEAO A2

•  E. Boldt was the PI of the HEAO A2 instrument, which was specifically designed to measure diffuse X-ray emission. •  Launched in 1977, it scanned the sky for ~17 months. •  It produced the first low-background, all-sky maps in the 2-60 keV

band.

HED 1 MED: l=±128° b=±45°

B E

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HEAO A2

•  E. Boldt led the group that produced key papers on unresolved X-ray emission. •  (Extragalactic) XRB spectrum (Marshall et al. 1980). •  (Extragalactic) XRB fluctuations (Shafer 1983). •  Thick disk GRXE (Iwan et al. 1982).

•  Pillbox model with h > 1500 pc; R > 14 kpc; kT ~ 9 keV. •  Due to halo stars? Hot gas?

•  Thin disk GRXE (Worrall et al. 1982). •  h ~ 241 pc; R ~ 3.5 kpc; •  Worrall & Marshall (1983) concluded that low luminosity stars such as

CVs and RS CVn stars are the most important contributors.

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Current Status

•  Recent missions provided better measurements. •  RXTE provided larger detectors and a smaller FOV (~1º). •  Chandra took deep images of a small region. •  Tenma and ASCA found a strong Fe-K line, which has now been

resolved with Suzaku.

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RXTE - I

•  Valinia & Marshall (1998) found 2 components with scale heights of 70 and 500 pc. Images cover l ∈ [56°,-44°] & |b| < 1.5° in 3 energy bands.

•  The spectrum was a 2-3 keV thermal plasma + power-law tail. •  Thermal emission from SN + PL from CR bremsstrahlung?

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RXTE - II

•  Revnivtsev et al. (2006) noted the strong correlation between the surface brightness in X-ray & NIR bands and thus the GRXE traces the stellar mass distribution.

•  They concluded that the bulk of the GRXE is due to weak X-ray sources such as CVs.

•  A sensitivity of 2x10-16 is needed to resolve 90% of the flux.

Contours: 3-20 keV X-ray

Image: 3.5 µm NIR

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Chandra Deep Images

•  Ebisawa et al. (2005) took deep (~100 ks) Chandra images from the Galactic Plane near l=28º.

•  Sensitivity is 3x10-15 (cgs). •  Few point sources beyond

expected from extragalactic. •  Point sources contribute small

fraction of total flux. •  High luminosity sources (Lx >

1033) cannot be significant contributors.

red: 0.5-2 keV; green: 2-4; blue: 4-8

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Better Spectra with Suzaku

•  Ebisawa et al. (2008) used Suzaku to resolve the Fe-lines from unresolved emission from the Galactic Plane near l=28.

•  Results indicate a multi-temperature plasma near collisional equilibrium. •  Cosmic-ray charge-exchange and non-equilibrium ionization

plasma models are inconsistent with the observed narrow lines and line ratios.

•  There is also an Fe fluorescence line.

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Future

•  Two approaches have been suggested to determine the main contributors to the GRXE.

•  Deep exposures to resolve most of the flux into discrete sources. •  Need to be able to detect sources at ~10-16 (cgs). •  Chandra could do this with very long exposures.

•  Elihu proposed this in a recent informal call for ideas for Chandra.

•  High resolution spectroscopy to determine the density of the emitting gas. •  Resolve He-like K-α triplet and compare strengths of forbidden

(6634 eV) and inter-combination lines (6680 & 6665 eV). •  Calorimeter on Astro H will be able to do this with very long

exposures. •  Elihu’s ideas inspired many (including myself) and led to several

decades of fascinating research.